In many industrial processes involving chemical solutions, the state of a chemical reaction may be measured by the potential (i.e., voltage) that develops between an inert electrode and a reference electrode immersed in the solution. This potential is known variously as the oxidation-reduction (ORP) or redox potential of the solution. In general, the potential that is measured reflects changes that are sensed by the inert electrode. The reference electrode ideally provides, as the name implies, a stable, constant reference point for the measured potential changes. In addition to stability and reproducibility of potential, practical applications of the ORP technique in industry require that both electrodes be mechanically rugged and resistant to corrosion by the process solutions.
Many devices have been designed and are commercially available for measuring ORP. These usually consist of an assembly consisting of both the inert, or indicating electrode, and a reference electrode. Indicating electrodes are almost always wires or disks of platinum or gold metal, or a less expensive substrate metal on which noble metals have been electroplated. Most reference electrodes embody half cells of the type metal/metal salt/solution, where the solution is in contact with the test solution whose redox potential is being measured. In this type of electrode, the salt is a sparingly soluble salt of the metal, and the solution contains anions of the salt. The most widely used examples of this type of electrode are the saturated calomel electrodes (SCE), with half cells having the composition Hg/Hg.sub.2 Cl.sub.2 /saturated KCl solution, and the silver/silver chloride electrodes, which is Ag/AgCl/solution saturated in AgCl and KCl (for example, U.S. Pat. Nos. 3,051,631 to Harbin and Munns, and 3,210,261 to Tyler). The reference electrode is usually fabricated so that its solution is in contact with the test solution at a porous junction. This junction, typically made of porous ceramic (for example U.S. Pat. Nos. 4,495,052 to Brezinski, and 4,495,053 to Souza) or plastic, fritted glass, or a small orifice, is selected to prevent excessive loss of the reference electrode solution, and mixing of it with the test solution, while maintaining a reasonably low electrical resistance for the potential measurement.
Most chemical processes and laboratory experiments requiring the measurement of ORP can employ reference electrodes found in the prior art, and which are commercially available. However, new chemical processes are being developed which involve the use of highly-concentrated, fuming nitric acids containing very strong oxidizing agents such as dinitrogen tetroxide, N.sub.2 O.sub.4, and dinitrogen pentoxide, N.sub.2 O.sub.5. These solutions include 100% HNO.sub.3, red fuming nitric acid, which is 20-40 wt. % N.sub.2 O.sub.4 in HNO.sub.3, and mixtures of HNO.sub.3, N.sub.2 O.sub.4, and N.sub.2 O.sub.5. Conventional reference electrodes are unsuitable for measurements of these solutions for several reasons. First, these nitric acid media are extremely aggressive toward the mechanical components of the electrode. Only glass, gold, the platinum metals, certain refractory metals such as tantalum, and certain fluorocarbon plastic materials are inert. Secondly, nitric acid media are especially reactive toward the half-cell metals, mercury and silver, of the conventional reference electrodes. Third, strong nitric acid and the nitrogen oxides react with chloride to form chlorine, which is detrimental to the functions of the reference electrode. Reference electrodes based on mercurous sulfate or oxide can be used to eliminate this problem, but the reactivity of mercury remains.
A type of reference electrode more suitable for use with strong nitric acid media is the redox type, which consists of an inert metal in contact with a solution containing the oxidized and reduced forms of an oxidation-reduction system. Examples of this type of electrode that have been used are gold with the Fe(III)/Fe(II) couple and platinum with iodine/iodide (U.S. Pat. No. 4,495,050, Ross). With equimolar concentrations of the oxidized and reduced forms of the couple, the potential of this reference electrode is established and buffered, or "poised" electrochemically, at a value corresponding to the formal potential of the couple in the electrode solution.
For compatibility with strong nitric acid media, the redox-type reference electrode should be based on solutions of nitric acid, and the redox couple chosen should be one whose formal potential is close to the potentials of the solutions to be measured. The redox couples used in the prior art, such as Fe(III)/Fe(II) and I.sub.2 /I.sup.- mentioned above, would be unstable in potential because the reduced species, Fe(II) and I.sup.-, would react rapidly with the nitric acid media.